Added resource specs to PBS executor, which were somehow missing.

[hategan]

Also split PBS executor into pbs_classic (the one using :ppn to specify processes per node) and the modern pbspro/openpbs (using select=:ncpus=).

Fixes #434

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (9e6abb9) 74.98% compared to head (5706789) 75.28%.
Report is 9 commits behind head on main.

Files	Patch %	Lines
src/psij/executors/batch/pbs.py	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #439      +/-   ##
==========================================
+ Coverage   74.98%   75.28%   +0.29%     
==========================================
  Files          92       94       +2     
  Lines        3794     3925     +131     
==========================================
+ Hits         2845     2955     +110     
- Misses        949      970      +21     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[andre-merzky]

I did not do a thorough review but also don't want to hold this up any longer. From what I have seen this looks sensible and clean. This will need to be tested on the respective schedulers - but I think a merge will only make tests more likely - so IMHO this is good to go.